Please check whether the use of 'einstall' in this ebuild is actually needed. From a quick glance at the Makefile, it seems to have proper support for DESTDIR. Therefore, if possible please replace the call to 'einstall' with proper 'default' or 'emake DESTDIR="${D}" install'. Thanks.
These wm* dockapps have old ebuilds, most should probably work fine with "classic" install commands. I'll check and update them Updated to 'emake DESTDIR="${D}" install'